Chapter 14
Telescopic Observation of the Moon
(Evening Observation)
Other than the Sun, the Moon is the most visible and intriguing object in the sky. Most
people are aware of the Moon’s effect on the tides; the ancient Babylonians even believed
that the sex of a child was determined by the phase of the Moon at conception.
In Aristotelian philosophy, the Moon—like all other astronomical objects—was believed to
be a geometrically-perfect sphere. In 1610, Galileo, having improved the newly invented
telescope, was able to report his observations of mountains and craters on the Moon, thus
refuting the Aristotelian view. He actually determined that some of the mountains were 4
miles high, which is very close to modern measurements. This discovery had a tremendous
influence on the decline of ancient science and the rise of the modern scientific method.
The Moon takes about 27.3 days to complete one orbit around the Earth, relative to the
distant suns. This is the Moon’s sidereal period1 . But, since the Earth is also moving, it
takes the moon a little longer—29.5 days—to go through a complete cycle of phases. This
is the Moon’s synodic period2 .
From a purely physical standpoint, the Moon orbits the Earth about once every 28 days. It
also rotates once about its own axis during this same period. As a result, the same side of
the Moon always faces Earth. This spin-orbit coupling is called a resonant orbit and is due
to tidal locking between the Earth and Moon.
In this lab, we’ll go up to the roof of Small Hall and use the Meade LX-10 8′′ telescopes to
take a look at some lunar features.
1
2
from the Latin word sidus, sideris, “star, constellation.”
from the Late Latin synodus, from the Greek synodos, “assembly, conjunction of planets.”
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14.1
Phases
The Moon goes through a number of phases over the course of each orbit. These phases
are not caused by the shadow of the Earth, but are due to the angle between the Moon,
Earth, and Sun. One-half of the Moon is always illuminated by the Sun, but it’s not always
the same half. As the Moon travels around the Earth, we see different amounts of that
illuminated half. Refer to Figure 14.1.
The cycle of phases begins with the new moon. This is when the Moon is positioned
between the Sun and the Earth, and the illuminated half of the Moon is on the Moon’s far
side. This is why the Moon cannot be seen during this part of the lunar cycle. From New,
the phases move through waxing crescent, first quarter, waxing gibbous, to full. As
the Moon is waxing, it is the western portion of the Moon that is illuminated.
The full moon occurs when the Earth is positioned between the Sun and the Moon. Next
come waning gibbous, third or last quarter, waning crescent, and then back again to
the new moon. As the Moon wanes, it is the eastern portion of the Moon that is illuminated
(refer to Figure 14.1). During its monthly orbit, the Moon moves about 13° eastward in the
sky each day, so the Moon rises and sets about an hour later each day. Convince yourself of
this.
Figure 14.1: The Cycle of the Moon.
14.2
Lunar Geography
The outer edge of the visible disk of the Moon is called the limb. The line of shadow that
crosses the surface, separating the lit and unlit portions, is called the terminator. Lunar
features are best observed when they are close to the terminator and cast long shadows over
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the surface. However some features, such as crater rays, are best distinguished when fully
illuminated such as when the Moon is full.
The names of lunar features often seem strange or obscure. The largest features, being the
easiest to see, were discovered and named soon after the invention of the telescope. Craters
were named after famous scientists and mathematicians, while the ‘seas’ were given fanciful
poetic names. Because this system was devised around 1650, many of the once-famous
persons immortalized by large craters are no longer well-known. Later scientists, such as
Einstein, got stuck with third-rate craters or had to wait for a place on the far side of the
Moon that cannot be observed from Earth.
Based on the examination of rocks brought back by the Apollo astronauts, it is now generally
believed that the Moon is composed of the same material as the Earth. For this reason, the
most popular theory of the Moon’s formation is that the Earth was struck by a Mars-sized
meteor while still in a molten state. The debris hurled upward by the impact created a
short-lived ring system. The debris eventually dispersed, but a large fraction coalesced to
form the Moon.
14.2.1
Physical Processes
The surface features of the Moon have been ascribed to two major causes. The first is
cratering—large or small bodies striking the surface and creating craters. Cratering activity
was most intense about 4.5 billion years ago, soon after the Moon formed and the early
Solar System was still filled with debris. The initial bombardment heated the lunar surface
and churned it violently. After a period of cooling, heat released by the radioactive decay of
elements in the Moon, melted the interior of the Moon giving rise to vulcanism. Vulcanism
is the release of molten material onto the surface by volcanoes or by welling up through deep
cracks. This was followed by a final cooling to its present frozen state. Neither mechanism
is active today.
Other minor processes are still at work modifying the surface on a small scale. Erosion
occurs from the constant rain of micro-meteors and solar particles blasting the surface. This
gradually wears features away and creates a fine powder on the surface. Erosion on the
Moon is exceedingly slow. In the last four billion years, not a single major surface feature
has been erased. By contrast, craters on the Earth have a lifetime of a few tens of thousands
of years. Moonquakes have been detected by seismometers left behind by Apollo astronauts.
They are low-energy events, imperceptible without instruments. No definite changes have
been seen on the Moon in four centuries of observations.
In this lab, you will look at different features of the Moon and draw them on an observation
sheet. The lunar features are usually divided into three categories—maria, craters, and
highlands. The following will describe each of these features for use in your drawings of the
Moon.
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14.2.2
Lunar Features
Maria
These are the dark areas on the Moon that look like ‘seas’. In Latin, the word mare means
sea3 . Maria are lowland, flat plains that resulted from the flow of lava from the Moon’s
interior when one or more large impactors struck the Moon and broke through the crust.
Craters
Virtually all of the Moon’s craters were formed by meteoroid impacts about 4 billion years
ago, in a heavy bombardment period. They are usually circular in shape and can vary in
size. The majority of craters seen through a telescope is concentrated towards the highlands
and is very old. A few craters are visible in the maria which makes them younger than the
event that formed the maria.
Highlands
These are the mountain-like structures on the Moon. They are the oldest part of the Moon’s
surface and were not formed by tectonic activity like the mountains on Earth. The highlands
were formed by millions of meteorite craters, one on top of the other, that have pushed the
top layer of the Moon’s surface upward. These features are named after mountains and
mountain ranges on Earth.
Refer to the Lunar Map on the wall and pick out some features that you’ll look at with the
telescope. Specifically, you’ll want to choose at least one of each type.
When everyone is ready, your TA will demonstrate how to set up the telescope, and you’ll
be on your way!
14.3
Tonight’s Procedure
14.3.1
Viewing with the naked eye
The first thing you will need to do is familiarize yourself with how the Moon looks in the sky.
Look at the Moon with your eyes. Draw what you see on the sheets provided. Use a pencil!
Be sure to indicate areas that are dark and areas that are light. Indicate on your drawings
what you think the light and dark areas are. Do you see any craters? If so, indicate these
as such on your drawing (label your drawing!).
14.3.2
Viewing with the telescope: 25 mm eyepiece
Next you will be looking at the Moon through the telescope. First use the 25 mm eyepiece,
which has the largest field of view. You will need to use the Neutral Density (ND) filter,
3
c.f. maritime, mariner, submarine, mermaid, etc.
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otherwise the light from the Moon will be too bright and it will be difficult to observe. This
filter screws on the end of the eyepiece. What do you see?
Draw the Moon as you see it through the telescope. Be sure to include as much detail as
you can. Where are the maria? Where are the highlands? Where do you see craters? When
your drawings are finished, compare the two. Is there a difference between the picture you
drew with your eyes, and through the telescope? If there is, explain what the differences are
and what you believe created this difference.
14.3.3
Viewing with the telescope: 15 mm eyepiece
Next, you will draw detailed pictures of three different areas of the Moon. This part of the
lab requires you to use a 15 mm eyepiece. The new eyepiece will give you a smaller field of
view and a more magnified view of the Moon. Scan the Moon and chose three areas that
predominately contain the three different features mentioned above. If you are unsure of the
areas, ask your TA for assistance.
For each of the different locations on the Moon, draw as detailed a picture as possible.
Include as many craters, maria, and highlands as you can, correctly placing them on your
drawing. Be careful to recognize the relative positions of the different features. You should
be able to notice certain things: some craters may be on top of others, other craters may be
filled in and look like the surrounding maria, the maria itself may have craters in it, there
may be many highland areas or there may be none. These are all things you will want to
make note of as you draw your pictures and make careful notes on what you see—you will
be using these in the next part of the lab.
After lab, compare your drawings to the Moon poster on the wall in your lab room. Try to
identify by name as many features as you can and write them on your observing sheet.
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Drawing of the Moon by Eye
Be sure to include the terminator. Indicate on your drawing what you think the light and
dark areas are. Do you see any craters? If so, indicate these as such on your drawing.
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Drawing of the Moon using the 25mm Eyepiece
When you are finished, compare this drawing to the previous one.
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Area 1—Note the position of your region on the smaller circle.
Draw a detailed diagram here. Be sure to make careful notes:
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Area 2—Note the position of your region on the smaller circle.
Draw a detailed diagram here. Be sure to make careful notes:
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Area 3—Note the position of your region on the smaller circle.
Draw a detailed diagram here. Be sure to make careful notes:
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Post-Observation Summary
This lab was designed for a number of reasons. It gives us a chance to use the telescopes
to look at stuff. We get to take a look at the Moon and get an idea of its geography. We
can learn to identify the phase of the moon by considering the direction that the bright limb
faces.
After completing this lab, you should be able to:
• Explain the accepted theory describing the Moon’s origin
• Know the difference between synodic and sidereal periods
• Know how to get a more magnified image of an object (i.e., know which eyepiece lens
to use)
• Use the telescope to locate the three types of land features on the Moon
• Use the telescope to locate the terminator
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